1. Technical Field
The present invention relates to a method and the like for determining the occurrence of a cross correlation.
2. Related Art
GPS (Global Positioning System) is widely known as a positioning system which uses a positioning signal and is used in a receiver that is provided in a mobile phone, a car navigation device, and the like. In the GPS, a position calculation computation is performed so as to calculate the position coordinates and timepiece errors of the receiver based on information such as the positions of a plurality of GPS satellites and a pseudo-range from each GPS satellite to the receiver.
A GPS satellite signal transmitted from a GPS satellite is modulated with spread codes, called C/A (Coarse and Acquisition) codes, different for each GPS satellite. The C/A codes are pseudo-random noise codes in which one PN frame has a code length of 1023 chips and which are repeated at a cycle of 1 ms. In order to acquire a GPS satellite signal from a weak reception signal, the receiver performs an operation of calculating a correlation between the reception signal and replica C/A codes which are pseudo C/A codes generated locally by the receiver to thereby acquire a GPS satellite signal based on a correlation value obtained as a result of the correlation.
Since the C/A codes are so-called Gold codes, a high correlation value is not obtained unless a correlation is calculated between a reception signal and a replica C/A code of a correct GPS satellite signal. However, the overall magnitude of the obtained correlation value can vary depending on the reception conditions of GPS satellite signals. Thus, for example, when the signal strength of a reception signal is strong, even if a correlation is calculated between the strong reception signal and a replica C/A code of the wrong satellite, a correlation value having a predetermined value or more is obtained. Thus, it is determined that a correlation is achieved. This is the case where a cross correlation occurs.
In this specification, a correlation when a satellite assumed by a GPS receiver is identical to an actual satellite will be defined as “correct correlation”. In contrast, a correlation when a satellite assumed by a GPS receiver is different from an actual satellite will be defined as “cross correlation”. When a cross correlation occurs, there is a problem in that the accuracy of position calculation decreases. Thus, various techniques for determining the occurrence of a cross correlation have been proposed (for example, see JP-A-2003-84055).
In the technique disclosed in JP-A-2003-84055, the occurrence of a cross correlation is determined by performing threshold determination on correlation values obtained by a correlation operation in a stepwise manner. In the technique disclosed in JP-A-2003-84055, it is necessary to appropriately set the threshold steps in order to make a cross correlation determination. However, reception conditions such as the signal strength and the reception environment when the receiver receives GPS satellite signals change from time to time. Therefore, there is another problem in that making a cross correlation determination in accordance with uniform determination criteria does not guarantee the reliability of determination.